Measurement While Drilling (MWD) directional sensors may be used to determine the orientation of wells during drilling operations. The MWD directional sensor may be packaged in a section of a drilling string near a drill bit.
Generally, a MWD directional sensor may include multiple magnetometers and accelerometers. The magnetometers may be used to measure components of the Earth's magnetic field (i.e., geomagnetic field) along three orthogonal axes related to the drill string section where the MWD directional sensor is located. These axes form the directional sensor frame. The Earth's gravity components along the same axes may be measured by the accelerometer. An accelerometer at rest on Earth measures along its axis the acceleration due to gravity. This acceleration component is same in magnitude as that of gravity but opposite in sign. The measurements of the magnetic and gravity field vectors at a survey point in a well may then be used to derive the orientation of the drill string and the well.
Generally, the measurement axes of magnetometers and accelerometers may be made to substantially align with each other, and used as axes of the directional sensor frame. The three accelerometers measure the Earth's gravity components along the mutually orthogonal X, Y, and Z-directions, where Z is along the drill string axis and X is nominally aligned with the hardware x-accelerometer. The magnetometers provide magnetic field components along the same axes (i.e., Z is along the drill string axis and X is nominally aligned with the hardware x-accelerometer). These three directions are the axes of the directional sensor frame.
Misalignments between the hardware axes and the directional sensor frame may be determined through a calibration process. In addition to misalignments, scale factor and bias of each field component sensor may also be determined from the calibration process.
The calibration parameters are known as calibration coefficients. The collection of all of the coefficients is called a calibration matrix. In operation, calibration coefficients are applied to hardware sensor outputs to obtain magnetic and gravitational field components along axes of the directional sensor frame.
In the current art, calibration is often determined using the magnitudes of Earth's magnetic field and gravitational field. Such techniques are known as the Total Field Calibration (TFC) methods. These techniques, however, may only be effective for calibration when the geomagnetic field is stable. As such, the current TFC calibration methods may be inaccurate if the geomagnetic field is changing.